1. Field of the Invention
This invention relates to high spatial resolution, X-ray computed tomography (ct) systems.
2. Background Art
The current state of imaging modalities used in these areas of dentomaxillofacial practice, with a particular emphasis on implantology, is described in the following subsections.
Implantology: Pre-operative and Post-operative Imaging
The use of dental implants is becoming an increasingly common treatment to replace missing teeth. Successful outcome of the treatment, osseointegration of the implant, depends heavily on precise presurgical planning. Since the functional load in implants can be high, it is important that the implant be placed in a position where it can contact cortical bone and at an angle where the forces are as perpendicular as possible. Selection of the appropriate size and inclination of the implant in both a bucco-lingual and mesio-distal direction requires precise knowledge of the anatomy of the proposed site, including its dimension in all planes, the presence of knife-edge ridges and undercuts, as well as the location of anatomic structures, such as the nasal fossae, the maxillary sinus, and the mandibular canal. An evaluation of the thickness of the cortical bone and the density of the medullary bone is also important to the success of the implant.
Acquiring the information needed for implant treatment planning requires some type of imaging examination. A number of imaging modalities have been used over the years, but they all have limitations and none are completely satisfactory.
Periapical radiography has enough resolution to depict trabecular bone pattern and the floor of the maxillary sinuses, but the images are limited in size and anatomic coverage and often suffer from geometric distortion. In addition, periapical radiographs are only two-dimensional images and do not provide information about the third, bucco-lingual, dimension.
Panoramic radiography is commonly used for implant site assessment because it is readily available, inexpensive, and provides wide coverage of the jaws. However, there are a large number of disadvantages to this technique that limit its usefulness for implant site assessment. The two primary problems are lack of information in the bucco-lingual plane and variability of the degree of magnification in different parts of the image due to changing distance between the rotational center, jaw structures, and film as the X-ray beam rotates around the head. Minor errors in positioning the patient's head in the machine can also exaggerate the degree of enlargement variability, particularly in the horizontal direction. Although dentists frequently try to overcome these magnification problems by having the patient wear a surgical stent with metal markers of a known size during the examination, this device is not adequate to solve all distortion errors.
Conventional (i.e., non-computed, “focal-plane” or “linear”) tomography has found an important application in the presurgical examination of proposed implant sites. Its major advantage over periapical and panoramic radiography stems from its ability to show bucco-lingual cross-sectional images. From these cross-sectional images, the dentist can estimate the spatial relationships of anatomic structures, bone height and width, and the inclination of the alveolar processes at the areas into which the implants are to be placed. Numerous studies have demonstrated that conventional tomography can depict the location of important anatomic structures, such as the mandibular canal, more accurately than panoramic radiography. Dimensional measurements are generally reliable with complex-motion tomography, and less so with linear-motion systems.
Conventional tomographs, however, have been reported not to be of diagnostic quality sufficient to allow identification of the canal in as many as 20% of cases. This is primarily due to the unavoidable blur that is inherent in the method, although some canals are poorly corticated and thus would be difficult to visualize with any technique. Another very important disadvantage of conventional tomography is that it is usually necessary to acquire multiple slices to ensure that the region of diagnostic interest is sampled adequately. Because each slice is acquired successively, the process is time-consuming and laborious, thus expensive, and it exposes the patient to a radiation dose that can be high, depending on the number of slices obtained.
X-ray computed tomography (CT) is a more sophisticated method for obtaining cross-sectional images than conventional tomography. It has been considered to be the most reliable technique for the assessment of bone height and width and localization of the inferior alveolar canal, mental foramen, nasopalatine canal, and maxillary sinuses. Consequently, it has been widely recommended for implant planning.
Conventional CT has its drawbacks, however. First, with its merely 2-D reformatted images, it may not clearly depict the inferior alveolar canal. Second, it is time-consuming: conventional CT requires more than 20 minutes to get all the axial slices for a dental implant study. With such a long scanning time, patient fatigue and patient swallowing start blurring the image. Third, conventional CT exposes the patient to a high radiation dose. Fourth, it is expensive. Fifth, it suffers from poor resolution, especially in the z-direction. Sixth, metal streak artifacts can occur in the presence of metallic dental restorations, requiring judicious selection of scan orientation and boundaries to minimize its occurrence.
Spiral CT is one of the most advanced imaging modalities available and is gradually replacing conventional CT. It is primarily used in the areas of medicine that require full body imaging, but is finding its use in dentistry as well. Spiral CT can generate not only 2-D cross-sectional images, but also fully 3-D images. Its 3-D capability is due to the fact that the X-ray source and detector continuously move along a spiral path relative to the body, thus acquiring data that are essentially 3-D. Spiral CT has been successfully used for the presurgical assessment for implant treatment planning.
However, the use of spiral CT in dentistry is hampered by its high cost and radiation dose, low spatial resolution in axial direction, and metal streak artifacts. In order to overcome some of the disadvantages, specifically high cost and radiation dose, two dental imaging methods have been proposed: Tuned-Aperture Computed Tomography (TACT™) and Ortho-CT.
TACT™, “an inexpensive alternative to CT,” is based on the theory of tomosynthesis. The relatively low cost of TACT is party due to its simplicity and partly due to its use of equipment that already exists at the facility. The use of TACT in implantology has been suggested, but no controlled studies have yet been performed. Nevertheless, its disadvantages in implant planning can be assessed from its characteristics. First, TACT requires the use of fiducial markers to estimate the imaging geometry and perform the reconstruction. This adds to the complexity of operating the instrument. Second, it uses CCD sensors of a low contrast resolution. Third, TACT does no have actual 3-D capabilities, but so-called pseudo-3D; 2-D images are being displayed from different angles, simulating varying projection geometries and providing some perception of three dimensions to the viewer. So, with TACT, the gain in cost-effectiveness is offset by a lower quality scan.
Ortho-CT is another potentially inexpensive alternative to spiral CT. Ortho-CT is basically a small cone-beam CT unit obtained by modifying a maxillofacial radiographic unit called Scanora® (Soredex, Helsinki, Finland) in order to acquire a “partial” CT scan.
In this context, the term “partial CT scan” refers to a cone-beam CT scan using a circular orbit, which, in addition to having the usual cone-beam incompleteness problem (i.e., a circular orbit does not satisfy Tuy's completeness conditions), is more incomplete in the sense that the data is insufficient for quantitatively reconstructing 3-D cross-sectional images to the use of a detector and scan geometry that do not measure all necessary rays through the object.
As an example, the “fan-beam scan” or central slice of the circular-orbit cone-beam scan is complete if the detector and scan geometry measure all parallel rays through the object at angles ranging from 0 to 180 degrees.
In the Ortho-CT device, a complete set of parallel rays is not measured at any view angle. It has a good spatial resolution; the resolving power at an MTF of 0.5 can be 1 lp mm−1 and the visual resolution limit about 2.0 lp mm−1. Radiation dose is low; skin dose is almost the same as with panoramic radiography and several dozen times lower than with conventional CT.
However, the applicability of Ortho-CT in implant planning is limited for four reasons. First, it can image only small areas (32×38 mm). If a larger area needs to be imaged, multiple scans are required. Second, its contrast resolution is so low that Ortho-CT is incapable of discriminating soft tissue. Third, its values are only relative and do not correspond to the absolute values of bone density. This is a consequence of its use of incomplete data. Fourth, although its behavior in the presence of metal fillings has not been reported on, it is expected that it suffers from metal streak artifacts.
In addition to presurgical planning of implants, there is a need for long-term maintenance and monitoring of tissue health around the implant after surgery has been performed. Peri-implantitis can progress around dental implants in a manner similar to the progression of periodontitis around natural teeth. If the determination of a failing implant could be made before the implant actually fails, therapeutic intervention might prevent further deterioration of implant support and loss of the implant. A long-standing goal of periodontal research is to find a diagnostic tool with a high sensitivity for detecting subtle disease activity around teeth. In particular, the detection of subtle changes in bone mass may be of great value for evaluating progressive periodontal disease or bone gain/loss after therapy.
Conventional radiography is routinely used by periodontists for the postsurgical assessment of the implant. It can display the mesial and distal aspects of the implant site, but it provides little information about the facial and lingual aspects of the implant site because of the obscuring effects of the radiopaque implant material. Its other major limitations include the subjective interpretation of the radiographic image, lack of sensitivity, and the inability to quantify bone mass.
Digital subtraction radiography can quantify bone mass and thus can be used for postsurgical implant assessment. The method involves taking two separate radiographic scans and then subtracting them. The two images are made at different times and must be as identical as possible. The exposure factors and processing parameters that affect density and contrast must be consistent between the two scans and the projection geometry must be duplicated as nearly as possible All this makes the method laborious and inefficient, especially when the medium is film, which is often the case. Finally, it cannot overcome the obscuring effects of a metal implant sufficiently to allow reliable detection of facial and lingual bone loss at implant sites.
The Temporomandibular Joint (TMJ)
When a patient presents complaints referable to the TMJ region, the findings from the clinical examination may indicate the need for imaging to aid in diagnosis and treatment planning. The osseous structures can be visualized with a variety of imaging techniques, including plain and panoramic radiography, conventional tomography and CT, depending on the degree of detail required.
Conventional tomography plays a significant role in TMJ imaging. Tomographic studies in the lateral and coronal planes demonstrate osseous components of the joint, whereas arthrotomographic examinations provide information about the status of the soft-tissue intra-articular disk. However, tomography of the TMJ is technically demanding because the imaging protocol must be customized for each patient due to the variability of condylar angles. Images made with linear-motion machines may also be suboptimal due to streaking artifacts and incomplete blurring of adjacent structures.
The use of computed tomography for TMJ imaging has generally been reserved for complex cases as a result of its relatively high cost and high radiation dose. Evaluation of articular disk position and function is usually performed with magnetic resonance imaging or arthrotomography, again both expensive techniques.
Detection of Facial Fractures
Another important problem in dentistry is the determination of the location and displacement of facial fractures in patients who have suffered trauma to the maxillofacial region. Complex trauma to the facial skeleton requires both highly qualified clinical knowledge and an accurate imaging technique.
Conventional radiography is unsuitable for the task because it requires multiple scans to obtain all the views that are necessary for evaluation. Even with a series of radiographs, it is sometimes difficult to detect subtle fractures. And conventional tomography, while theoretically capable of demonstrating complex anatomy better than planar projection radiographs, has generally been superseded by CT in most hospitals.
Spiral CT imaging is a better solution for detection of facial fractures. Only one spiral CT scan is needed for the examination, and it does not require movement of the patient to obtain multiple views. It is very fast and can scan the entire midface and the frontal sinus in less than a minute. Thus, it allows the diagnostician to move on to other essential diagnostic and therapeutic interventions without delay. Also, it allows for further processing of the data without requiring the patient's continued presence in the CT unit. Finally, spiral CT produces images of superior quality and can be used to generate 3-D images that can be rotated on a video screen to demonstrate the anatomy and pathology from all angles.
However, the use of spiral CT for detecting facial fractures has drawbacks. It is costly and it exposes the patient to a high radiation dose. It also suffers from metal streak artifacts, which can result in misleading scans of the facial complex.
Lesions and Diseases of Soft Tissue in the Head and Neck
In addition to imaging of the bony structures of the maxillofacial complex, a very important task is the imaging of the soft tissues in the head and neck. Particularly important is imaging of inflammations, cysts, and tumors.
While conventional radiography produces adequate bone images, it provides little information regarding soft tissues as a result of the inability of film-screen systems to record X-ray attenuation differences of less than 2%.
CT, however, is much more effective at separating subtle tissue contrast difference (as low as 0.5%). CT can differentiate not only soft tissue from bone, but also various types of soft tissues from each other. Consequently, CT has found a very important application in the evaluation of the presence and extent of clinically suspected pathology in the head and neck, including tumors, cysts, and inflammations. When additional information concerning the soft tissues is required, an intravenous contrast agent can be used. Cavalcanti et al. have successfully used spiral CT to measure the volume of oral tumors.
Although CT has proven superior in many aspects to other modalities in this application, its use has been limited by three factors: cost, radiation dose and metal streak artifacts. In addition, the spatial resolution available in current CT scanners, which are designed primarily for full-body imaging, may not be optimal for lesions in the head and neck.
MRI has also been used to image soft tissue of the head and neck. Some advantages of MRI over CT include better contrast resolution, absence of artifact degradation from dental restorations, visualization of major vessels without intravenous injection of contrast material, and direct, three-plane imaging without patient repositioning. MRI, however, is more expensive than CT, and requires a longer time to obtain a scan. To date, there is no general consensus about which imaging technique is optimal for use in diagnosis of lesions in the head.
Reconstructive Facial Surgery
Surgery of craniofacial deformities is a complex task that requires careful preoperative planning and specific, detailed information on patient's pathology and anatomy. The goals of maxillofacial surgery are not limited to treating the condition of bone, but extend to improving both the morphology and function of soft tissues, such as facial appearance for the patient. It is necessary, therefore, to ascertain how the proposed alteration of bone will affect the form and function of the surrounding soft tissue. Effective planning of reconstructive facial surgery requires not only adequate imaging of the bone and soft tissue but also the means of interpreting these images in combination to predict how the surgical alteration of bone will affect soft tissue.
Radiography has been used with a certain amount of success for planning facial reconstructive surgery. However, facial reconstructive surgery presents a three-dimensional problem of anatomical rearrangement and cannot be effectively planned using two-dimensional images. Morever, some facial and skeletal anomalies, specifically those involving facial asymmetry, are not amenable to analysis using only two-dimensional images.
CT and MRI can supply detailed, three-dimensional information on the patient's anatomy, and have therefore become the methods of choice in maxillofacial surgery planning. They are typically used in conjunction with computer software that shows the predicted three-dimensional rendered postoperative facial surface. However, both are expensive. In addition, CT suffers from metal streak artifacts and exposes the patient to a high radiation dose while MRI is not particularly useful for examining bony structures.
Several devices have been conceived for dentomaxillofacial imaging. Ortho-CT is a vertically oriented device that acquires a partial CT scan, and while spatial resolution is adequate, the device suffers from artifacts due to the incomplete nature of the scan and does not produce quantitatively accurate estimates of attenuation (necessary for assessing bone quality).
TomCAT is a cone-beam imaging device in which the patient lies supine on an imaging table similar to conventional spiral or single-slice CT instruments. The TomCAT uses an image intensifier and CCD camera for the detector—a combination that has relatively poor dynamic range (reducing quantitative accuracy) and also suffers from spatial distortions. Although images reconstructed from TomCAT have spatial resolution similar to conventional general-purpose CT instruments, contrast resolution is quite poor and images appear to suffer from a great deal of X-ray scatter (and perhaps veiling glare from the image intensifier).
Individual methods are known to be close to the techniques described herein (e.g., Wagner's method for scatter estimation and correction).
The following U.S. patents are deemed to be relevant to the present invention: U.S. Pat. Nos. 5,390,112; 5,615,279; 6,018,563; 5,909,476; 6,118,842; 6,075,836; and 5,999,587. The following U.S. patents are deemed to be of less relevance to the present invention: U.S. Pat. Nos. 5,927,982; 6,094,467; 6,125,193; 5,461,650; 4,812,983; 4,590,558; 4,709,333; 5,243,664; 5,798,924; 6,035,012; 5,293,312; 5,390,112; 5,615,279; 5,644,612; 5,751,785; 5,042,487; and 5,909,476. The following U.S. patents are also deemed to be relevant: U.S. Pat. Nos. 5,778,045; 5,793,838; 5,805,659; 5,815,546; 5,864,146; 5,878,108; 5,881,123; 5,903,008; 5,921,927; 5,949,846; 5,970,112; 5,995,580; 6,052,428; 6,101,234; 6,101,236; 6,104,775; 6,118,841; 6,185,271B1; 6,285,733B1; 6,285,740B1; 6,289,074B1; 6,292,527B1; 6,298,110B1; 6,324,246B1.